Low-cost austenitic stainless steel having high strength and high formability, and method for manufacturing same

ABSTRACT

Disclosed are a low-cost austenitic stainless steel having high strength and high formability and a method for manufacturing same. The low-cost austenitic stainless steel having high strength and high formability according to an embodiment includes, greater than 0% and at most 0.08% of C, 0.2 to 0.25% of N, 0.8 to 1.5% of Si, 8.0 to 9.5% of Mn, 15.0 to 16.5% of Cr, greater than 0% and at most 1.0% of Ni, 0.8 to 1.8% of Cu, and the remainder of Fe and other unavoidable impurities and satisfies Expressions (1) to (4) below.Ni+0.47Mn+15N≥7.5  (1)23(C+N)+1.3 Si+0.24(Cr+Ni+Cu)+0.1Mn≥12  (2)551−462(C+N)−9.2Si−8.1Mn−13.7Cr−29(Ni+Cu)≤70  (3)11≤1+45C−5Si+0.09Mn+2.2Ni−0.28Cr−0.67Cu+88.6N≤17  (4)Here, C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt %) of the elements, respectively.

TECHNICAL FIELD

The present disclosure relates to an austenitic stainless steel and amethod for manufacturing same, and more particularly, to a low-costaustenitic stainless steel having high strength and high formability anda method for manufacturing same.

BACKGROUND ART

Vehicle market trends are changing from conventional internal combustionengine-based automotive industry toward battery-based eco-friendlyvehicle markets. That is, conventional internal combustion enginevehicle markets which are of high interest in middle-sized orlarge-sized vehicles are changing toward battery-based vehicle marketswhich prefer small-sized or lightweight vehicles.

Structural materials protecting batteries are required to have highstrength in order to protect the batteries from the risk of safetyaccidents such as explosions or from external impact and for the safetyof passengers, and the structural materials are also required to belightweight to prevent weight of small-sized or lightweight vehiclesfrom increasing. As well as structural materials for protectingbatteries, general structural materials have become smaller in size andhigher in strength to comply with environmental regulations.Accordingly, there is a need to develop materials with highproductivity, excellent stability, high strength, and excellentformability applicable throughout the industry.

Stainless steels are materials applicable throughout the industry due toexcellent corrosion resistance. Particularly, austenitic stainlesssteels with excellent elongation have no problem in forming complexshapes to meet various needs of customers and are advantageous in termsof aesthetic appearance.

However, austenitic stainless steels have lower yield strength comparedto common carbon steels and are economically disadvantageous becauseexpensive alloying elements are used therein. Therefore, there is a needto develop stainless steels for structural materials having high levelsof yield strength and proper tensile strength with excellent formabilitymaintained.

In addition, there is a problem in that alloying elements constitutingaustenitic stainless steels are expensive compared to elementsconstituting most carbon steels. Particularly, Ni included in austeniticstainless steels may cause problems in terms of price competitivenessbecause it is expensive and difficult to stably supply Ni due tounstable supply and demand thereof due to a wide fluctuation in prices.Therefore, there is a need to develop low-cost austenitic stainlesssteels in which the contents of expensive elements such as Ni arereduced.

DISCLOSURE Technical Problem

To solve the above-described problems, provided is a low-cost austeniticstainless steel having high strength and high formability.

Technical Solution

In accordance with an aspect of the present disclosure to achieve theabove-described objects, a low-cost austenitic stainless steel havinghigh strength and high formability includes, in percent by weight (wt%), greater than 0% and at most 0.08% of C, 0.2 to 0.25% of N, 0.8 to1.5% of Si, 8.0 to 9.5% of Mn, 15.0 to 16.5% of Cr, greater than 0% andat most 1.0% of Ni, 0.8 to 1.8% of Cu, and the remainder of Fe and otherunavoidable impurities and satisfies Expressions (1) to (4) below:

Ni+0.47Mn+15N≥7.5  (1)

23 (C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn≥12  (2)

551−462(C+N)−9.2Si−8.1Mn−13.7Cr−29(Ni+Cu)≤70  (3)

11≤1+45C−5Si+0.09Mn+2.2Ni−0.28Cr−0.67Cu+88.6N≤17  (4)

wherein C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt %) of theelements, respectively.

In each low-cost austenitic stainless steel having high strength andhigh formability of the present disclosure, a yield strength of acold-rolled, annealed steel sheet may be 400 MPa or more.

In each low-cost austenitic stainless steel having high strength andhigh formability of the present disclosure, an elongation of acold-rolled, annealed steel sheet may be 55% or more.

In each low-cost austenitic stainless steel having high strength andhigh formability of the present disclosure, a yield strength of a skinpass-rolled steel sheet may be 800 MPa or more.

In each low-cost austenitic stainless steel having high strength andhigh formability of the present disclosure, an elongation of the skinpass-rolled steel sheet may be 25% or more.

Also, in accordance with an aspect of the present disclosure to achievethe above-described objects, a method for manufacturing a low-costaustenitic stainless steel having high strength and high formabilityincludes: preparing a slab including, in percent by weight (wt %),greater than 0% and at most 0.08% of C, 0.2 to 0.25% of N, 0.8 to 1.5%of Si, 8.0 to 9.5% of Mn, 15.0 to 16.5% of Cr, greater than 0% and atmost 1.0% of Ni, 0.8 to 1.8% of Cu, and the remainder of Fe and otherunavoidable impurities and satisfying Expressions (1) to (4) below; hotrolling the slab to prepare a hot-rolled steel sheet and hot annealingthe hot-rolled steel sheet to prepare a hot-rolled, annealed steelsheet; cold rolling the hot-rolled, annealed steel sheet to prepare acold-rolled steel sheet and cold annealing the cold-rolled steel sheetat a temperature of 1050° C. or higher to prepare a cold-rolled,annealed steel sheet; and skin pass rolling the cold-rolled, annealedsteel sheet to prepare a skin pass-rolled steel sheet:

Ni+0.47Mn+15N≥7.5  (1)

23 (C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn≥12  (2)

551−462(C+N)−9.2Si−8.1Mn−13.7Cr−29(Ni+Cu)≤70  (3)

11≤1+45C−5Si+0.09Mn+2.2Ni−0.28Cr−0.67Cu+88.6N≤17  (4)

wherein C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt %) of theelements, respectively.

In the method for manufacturing each low-cost austenitic stainless steelhaving high strength and high formability, the skin pass rolling may beperformed at a reduction ratio of 20% or more.

In the method for manufacturing each low-cost austenitic stainless steelhaving high strength and high formability, the slab may have a reductionof area of 50% or more at a high temperature of 800° C. or higher.

Advantageous Effects

According to an embodiment of the present disclosure, provided is anaustenitic stainless steels having excellent yield strength, in which acold-rolled, annealed steel sheet prepared by cold annealing at atemperature of 1050° C. or higher after cold rolling has excellent yieldstrength and excellent elongation sufficient for forming may be obtainedafter skin pass rolling performed to further increase strength. Also, alow-cost austenitic stainless steel having high strength and highformability with high productivity even using reduced amounts ofexpensive alloying elements may be provided.

BEST MODE

A low-cost austenitic stainless steel having high strength and highformability according to an embodiment of the present disclosureincludes, in percent by weight (wt %), greater than 0% and at most 0.08%of C, 0.2 to 0.25% of N, 0.8 to 1.5% of Si, 8.0 to 9.5% of Mn, 15.0 to16.5% of Cr, greater than 0% and at most 1.0% of Ni, 0.8 to 1.8% of Cu,and the remainder of Fe and other unavoidable impurities and satisfiesExpressions (1) to (4) below.

Ni+0.47Mn+15N≥7.5  (1)

23 (C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn≥12  (2)

551−462(C+N)−9.2Si−8.1Mn−13.7Cr−29(Ni+Cu)≤70  (3)

11≤1+45C−5Si+0.09Mn+2.2Ni−0.28Cr−0.67Cu+88.6N≤17  (4)

wherein C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt %) of theelements, respectively.

Modes of the Invention

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the accompanying drawings. The embodiments ofthe present disclosure may, however, be embodied in many different formsand should not be construed as being limited to the embodiments setforth herein. Rather, these embodiments are provided so that thisdisclosure will be thorough and complete, and will fully convey theconcept of the invention to those skilled in the art.

Also, the terms used herein are merely used to describe particularembodiments. An expression used in the singular encompasses theexpression of the plural, unless otherwise indicated. Throughout thespecification, the terms such as “including” or “having” are intended toindicate the existence of features, operations, functions, components,or combinations thereof disclosed in the specification, and are notintended to preclude the possibility that one or more other features,operations, functions, components, or combinations thereof may exist ormay be added.

Meanwhile, unless otherwise defined, all terms used herein have the samemeaning as commonly understood by one of ordinary skill in the art towhich this disclosure belongs. Thus, these terms should not beinterpreted in an idealized or overly formal sense unless expressly sodefined herein. As used herein, the singular forms are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise.

The terms “about”, “substantially”, etc. used throughout thespecification means that when a natural manufacturing and a substanceallowable error are suggested, such an allowable error corresponds thevalue or is similar to the value, and such values are intended for thesake of clear understanding of the present invention or to prevent anunconscious infringer from illegally using the disclosure of the presentinvention.

A low-cost austenitic stainless steel having high strength and highformability according to an embodiment of the present disclosureincludes, in percent by weight (wt %), greater than 0% and at most 0.08%of C, 0.2 to 0.25% of N, 0.8 to 1.5% of Si, 8.0 to 9.5% of Mn, 15.0 to16.5% of Cr, greater than 0% and at most 1.0% of Ni, 0.8 to 1.8% of Cu,and the remainder of Fe and other unavoidable impurities.

Hereinafter, reasons for numerical limitations on the contents ofalloying elements in the embodiment of the present disclosure will bedescribed.

Carbon (C): Greater than 0 wt % and at most 0.08 wt %

Carbon (C), as an element effective on stabilizing an austenite phase,is added to obtain a yield strength of an austenitic stainless steel.However, an excess of C may not only deteriorate cold workability due tosolid strengthening effect but also may induce grain boundaryprecipitation of a Cr carbide thereby adversely affecting ductility,toughness, corrosion resistance, and the like and deteriorating weldingproperties among the elements. Therefore, an upper limit thereof may beset to 0.08 wt %.

Nitrogen (N): 0.2 to 0.25 wt %

Nitrogen (N) is the most important element in the present disclosure.Nitrogen is a strong austenite-stabilizing element effective onenhancing corrosion resistance and yield strength of an austeniticstainless steel. However, an excess of N may cause occurrence of defectssuch as nitrogen pores while a slab is made and deteriorate coldworkability due to solid solution strengthening effect. Therefore, anupper limit thereof may be set to 0.25 wt %.

Silicon (Si): 0.8 to 1.5 wt %

Silicon (Si), acting as a deoxidizer during a steelmaking process, is anelement effective for improving corrosion resistance. Also, Si is aneffective element for increasing yield strength of steel materials amongsubstitutional elements. In consideration of these effects, Si may beadded in an amount of 0.8 wt % or more in the present disclosure.However, an excess of Si, as a ferrite phase-stabilizing element, maypromote formation of delta (δ) ferrite in a cast slab, thereby not onlydeteriorating hot workability but also deteriorating ductility andimpact characteristics of steel materials. Therefore, an upper limit ofthe Si content may be set to 1.5 wt %.

Manganese (Mn): 8.0 to 9.5 wt %

Manganese (Mn), as an austenite phase-stabilizing element added as a Nisubstitute, may be added in an amount of 8.0 wt % or more to enhancecold workability by inhibiting formation of strain-induced martensite.However, an excess of Mn causes an increase in formation of S-basedinclusions (MnS) leading to deterioration of ductility and toughnessaustenitic stainless steels and may cause formation of Mn fumes during asteelmaking process resulting in increased manufacturing risks. Also, anexcess of Mn rapidly deteriorates corrosion resistance of products.Therefore, an upper limit of the Mn content may be set to 9.5 wt %.

Chromium (Cr): 15.0 to 16.5 wt %

Chromium (Cr) is a ferrite-stabilizing element but effective onsuppressing formation of a martensite phase. As a basic element forobtaining corrosion resistance required in stainless steels, Cr may beadded in an amount of 15% or more. However, an excess of Cr, as aferrite-stabilizing element, may promote formation of delta (δ) ferritein a slab in large quantity resulting in deterioration of hotworkability and adverse effects on material characteristics. Therefore,an upper limit thereof may be set to 16.5 wt %.

Nickel (Ni): Greater than 0 wt % and at most 1.0 wt %

Nickel (Ni), as a strong austenite phase-stabilizing element, is addedto improve hot workability and cold workability. However, because Ni isan expensive element, costs of raw materials may increase in the case ofadding a large amount of Ni. Therefore, an upper limit of the Ni contentmay be set to 1.0% in consideration of both costs and efficiency ofsteel materials.

Copper (Cu): 0.8 to 1.8 wt %

Copper (Cu), as an austenite phase-stabilizing element added instead ofnickel (Ni) in the present disclosure. Also, Cu, as an element improvingcorrosion resistance of steel materials under a reducing environment,may be added in an amount of 0.8 wt % or more. However, an excess of Cunot only increases costs of steel materials but also causes liquefactionand embrittlement at a low temperature. Also, an excess of Cu may besegregated in edges of a slab, thereby deteriorating hot workability ofsteel materials. Thus, an upper limit of the Cu content may be set to1.8 wt % in consideration of costs, efficiency, and properties of steelmaterials.

The remaining component of the composition of the present disclosure isiron (Fe). However, the composition may include unintended impuritiesinevitably incorporated from raw materials or surrounding environments.In the present disclosure, addition of other unintended alloyingelements in addition to the above-described alloying elements is notexcluded. The impurities are not specifically mentioned in the presentdisclosure, as they are known to any person skilled in the art.

Examples of the unavoidable impurities include phosphorus (P) and sulfur(S), and at least one of P (at most 0.035 wt %) and S (at most 0.01 wt%) may be contained according to an embodiment of the presentdisclosure.

Phosphorus (P): at most 0.035 wt %

Phosphorus (P), as an impurity that is inevitably contained in steels,is a major causative element of grain boundary corrosion of steelmaterials or deterioration of hot workability, and therefore, it ispreferable to control the P content as low as possible. In the presentdisclosure, an upper limit of the P content may be set to 0.035 wt %.

Sulfur (S): at most 0.01 wt %

Sulfur (S), as an impurity that is inevitably contained in steels, is amajor causative element of deterioration of hot workability as beingsegregated in grain boundaries, and therefore, it is preferable tocontrol the S content as low as possible. In the present disclosure, anupper limit of S may be set to 0.01 wt %.

It is important to improve yield strength of steel materials to decreaseweight of the steel materials and enhance stability. In addition,sufficient elongation should be obtained to manufacture structuralmaterials having various shape including battery module cases. Inaddition, in order to obtain price competitiveness of austeniticstainless steels, the amounts of expensive austenite-stabilizingelements such as Ni need to be reduced and the amounts of elementsreplacing the expensive elements such as Mn, N, and Cu should beappropriately adjusted.

However, in the case where the Ni content is reduced and Mn, N, and Cuare added, work hardening is rapidly increased to deteriorate elongationof a steel material and induce reduction in resistance to hotdeformation, thereby deteriorating productivity, and thus harmony of therespective alloying elements should be considered. In consideration ofthe yield strength, elongation, and price competitiveness of steelmaterials as described above, the composition of alloying elements mayfurther be limited to satisfy Expressions (1) to (4) in addition to theabove-described composition.

In the present disclosure, in order to obtain excellent elongation of acold-rolled, annealed steel sheet prepared by cold rolling and annealingthe steel material, Expression (1) regarding a fraction of an austenitephase has been derived.

Ni+0.47Mn+15N≥7.5  (1)

Here, Mn, Ni, and N denote contents (wt %) of the elements,respectively.

As the value of Expression (1) decreases, the fraction of the austenitephase decreases. When the value of Expression (1) is less than 7.5, theaustenitic stainless steel may include delta ferrite in an amount of 5%or more or phase transformation into martensite phase occurs during coldrolling. As a result, elongation of the austenitic stainless steel maydeteriorate, and thus a lower limit of the value of Expression (1) maybe set to 7.5 in the present disclosure to obtain a sufficientelongation.

In addition, in order to obtain a high yield strength of the austeniticstainless steel, Expression (2) has been derived in the presentdisclosure in consideration that the yield strength is improved by astress field of a steel material.

23 (C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn≥12  (2)

Here, C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt %) of theelements, respectively.

As the value of Expression (2) increases, a stress field betweenlattices increases due to size difference among the alloying elements sothat a limit to withstand plastic deformation against external stressincreases. When the value of Expression (2) is less than 12, it isdifficult to obtain a yield strength required in the present disclosure.Therefore, a lower limit of the value of Expression (2) may be set to 12in the present disclosure to obtain high strength characteristics.

In addition, in consideration of phase transformation caused bydeformation of the austenitic stainless steel, Expression (3) has beenderived in the present disclosure.

551−462(C+N)−9.2Si−8.1Mn−13.7Cr−29(Ni+Cu)≤70  (3)

Here, C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt %) of theelements, respectively.

As the value of Expression (3) increases, the austenite phase is easilytransformed by an external stress. Specifically, when the value ofExpression (3) exceeds 70, the austenitic stainless steel exhibits arapid strain-induced martensite transformation behavior, causingnon-uniform plastic processing. As a result, a problem of deterioratingelongation of the austenitic stainless steel may occur, and thus a lowerlimit of the value of Expression (3) may be set to 70.

In addition, in consideration of dislocation slip behavior of steelmaterials due to deformation of the austenitic stainless steel,Expression (4) has been derived.

11≤1+45C−5Si+0.09Mn+2.2Ni−0.28Cr−0.67Cu+88.6N≤17  (4)

Here, C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt %) of theelements, respectively.

As the value of Expression (4) decreases, expression of cross slip of anaustenite phase by an external stress becomes difficult. When the valueof Expression (4) is less than 11, the austenitic stainless steelexhibits only a planar slip behavior with respect to deformation anddislocation is rapidly piled up by an external stress. As a result,problems of non-uniform plastic processing and high work hardening mayoccur. Accordingly, the elongation of the austenitic stainless steel maydeteriorate, it may be difficult to perform the skin pass rolling, andhot rolling defects such as edge cracks may occur during deformation ata high temperature, thereby causing a problem of decreasingproductivity. In consideration thereof, a lower limit of Expression (4)may be set to 11.

On the contrary, when the value of Expression (4) is too high, crossslip frequently occurs causing a problem of increasing plasticnon-uniformity in which a stress is concentrated in a weak part of asteel material. As strength of a steel material increases, suchembrittlement and plastic non-uniformity tend to increase, and thuselongation of steel materials having a high strength as in the presentdisclosure likely deteriorates. In consideration thereof, an upper limitof the value of Expression (4) may be set to 17.

Since Cr—Mn steels, in which the Ni content is reduced compared tocommercially available 300 series austenitic stainless steels, haveinferior hot workability, an actual yield may decrease due to occurrenceof edge cracks during a hot processing and correcting costs may increaseor there may be a need to invest additional equipment to reduce edgecracks. According to the present disclosure, excellent hot workabilitymay be obtained by satisfying the above-described composition ofalloying elements and appropriately designing the composition ofalloying elements using Expressions (1) to (4) without adding a separateprocess and equipment. According to an embodiment of the presentdisclosure, the slab having the above-described composition of alloyingelements may have a reduction of area of 50% or more at a hightemperature of 800° C. or higher.

In the low-cost austenitic stainless steel having high strength and highformability according to an embodiment of the present disclosure, ayield strength of a cold-rolled, annealed steel sheet may be 400 MPa. Inaddition, in the low-cost austenitic stainless steel having highstrength and high formability, an elongation of the cold-rolled,annealed steel sheet may be 55% or more. In this regard, the“cold-rolled, annealed steel sheet” refers to a steel material preparedby treating a slab by hot rolling, annealing, cold rolling, andannealing.

In the low-cost austenitic stainless steel having high strength and highformability according to an embodiment of the present disclosure, ayield strength of a skin pass-rolled steel sheet may be 800 MPa or more.In addition, according to an embodiment, particularly, a yield strengthmay be 800 MPa or more and an elongation may be 25% or more. In thisregard, the “skin pass-rolled steel sheet” refers to a steel materialprepared by skin pass rolling the above-described cold-rolled, annealedsteel sheet.

Hereinafter, a method for manufacturing the low-cost austeniticstainless steel having high strength and high formability according tothe present disclosure will be described.

The method for manufacturing the low-cost austenitic stainless steelhaving high strength and high formability according to an embodiment ofthe present disclosure includes: preparing a slab including, in percentby weight (wt %), greater than 0% and at most 0.08% of C, 0.2 to 0.25%of N, 0.8 to 1.5% of Si, 8.0 to 9.5% of Mn, 15.0 to 16.5% of Cr, greaterthan 0% and at most 1.0% of Ni, 0.8 to 1.8% of Cu, and the remainder ofFe and other unavoidable impurities and satisfying Expressions (1) to(4); hot rolling the slab to prepare a hot-rolled steel sheet and hotannealing the hot-rolled steel sheet to prepare a hot-rolled, annealedsteel sheet; cold rolling the hot-rolled, annealed steel sheet toprepare a cold-rolled steel sheet and cold annealing the cold-rolledsteel sheet at a temperature of 1050° C. or higher to prepare acold-rolled, annealed steel sheet, and skin pass rolling thecold-rolled, annealed steel sheet to prepare a skin pass-rolled steelsheet.

Reasons for numerical limitations on the contents of the alloyingelements and Expressions (1) to (4) are as described above. Hereinafter,each of the manufacturing steps will be described in detail.

The slab having the above-described composition of alloying elements maybe hot-rolled at a temperature of 1000 to 1300° C. to prepare ahot-rolled steel sheet, and then annealed at a temperature of 1000 to1100° C. to prepare a hot-rolled, annealed steel sheet. In this regard,annealing heat treatment may be performed for 10 seconds to 10 minutes.

Subsequently, the hot-rolled, annealed steel sheet is cold-rolled toprepare a cold-rolled steel sheet and then annealed to prepare acold-rolled, annealed steel sheet. Conventionally, as a method forimproving a yield strength of an austenitic stainless steel,low-temperature annealing heat treatment was performed at a lowtemperature of 1000° C. or below after cold rolling as described above.The low-temperature annealing heat treatment is a method for increasingstrength using energy accumulated in the steel material during coldrolling without completing recrystallization. However, in such anaustenitic stainless steel that has undergone low-temperature annealingheat treatment, under pickling may occur during a subsequent pickingprocess or aesthetic appearance may not be obtained as well as thepossibility of non-uniform quality.

According to an embodiment of the present disclosure, the hot-rolled,annealed steel sheet is cold-rolled to prepare a cold-rolled steelsheet, and then annealed at a temperature of 1050° C. or higher toprepare a cold-rolled, annealed steel sheet. In this case, the annealingheat treatment may be performed for 10 seconds to 10 minutes.

According to the present disclosure, excellent elongation may beobtained because low-temperature annealing is not performed after coldrolling, and an appropriate level of yield strength may be obtained bydesigning the composition of alloying elements.

The cold-rolled, annealed steel sheet according to the presentdisclosure may have a yield strength of 400 MPa or more.

The cold-rolled, annealed steel sheet according to the presentdisclosure may have an elongation of 55% or more.

By designing the composition of alloying elements as described above, acold-rolled, annealed steel sheet may have an appropriate yield strengthwithout performing low-temperature annealing treatment via a processwhich does not cause loads on production.

In addition, according to the present disclosure, high yield strengthmay be obtained via adjustment of the composition of alloying elementsand subsequent skin pass rolling without performing low-temperatureannealing treatment after cold rolling. According to an embodiment ofthe present disclosure, the yield strength of the skin pass-rolled steelsheet may be 800 MPa or more. The skin pass rolling may be performed ata reduction ratio 20% or more according to the present disclosure.

Skin pass rolling may increase strength by using a high work hardeningphenomenon while the austenite phase is transformed into strain-inducedmartensite during cold deformation or using dislocation pile-up of asteel material. However, elongation of the steel material may rapidlydeteriorate by skin pass rolling.

According to the present disclosure, a rapid decrease in elongation of asteel material, which is caused by skin pass rolling, may be preventedby appropriately controlling phase transformation and dislocationbehavior by designing the composition of alloying elements as describedabove. As a result, a low-cost austenitic stainless steel having highstrength and high formability, in which a skin pass-rolled steel sheethas a yield strength of 800 MPa or more and an elongation of 25% ormore, may be provided according to an embodiment of the presentdisclosure.

Hereinafter, the present disclosure will be described in more detailthrough examples. However, it is necessary to note that the followingexamples are only intended to illustrate the present disclosure in moredetail and are not intended to limit the scope of the presentdisclosure. This is because the scope of the present disclosure isdetermined by matters described in the claims and able to be reasonablyinferred therefrom.

EXAMPLES

Slabs having compositions of allying elements shown in Table 1 belowwere prepared by ingot melting, heated at 1250° C. for 2 hours, andhot-rolled to prepare hot-rolled steel sheets. Then, the hot-rolledsteel sheets were subjected to annealing heat treatment at 1100° C. for90 seconds to prepare hot-rolled, annealed steel sheets. Subsequently,the steel materials were cold-rolled at a reduction ratio of 70% toprepare cold-rolled steel sheets and subjected to annealing heattreatment at 1100° C. for 10 seconds to prepare cold-rolled, annealedsteel sheets.

Compositions of alloying elements of each of inventive examples andcomparative examples and values obtained by substituting the contents ofthe alloying elements into Expressions (1) and (4) are shown in Table 1below.

Ni+0.47Mn+15N≥7.5  (1)

23 (C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn≥12  (2)

551−462(C+N)−9.2Si−8.1Mn−13.7Cr−29(Ni+Cu)≤70  (3)

11≤1+45C−5Si+0.09Mn+2.2Ni−0.28Cr−0.67Cu+88.6N≤17  (4)

TABLE 1 Elements (wt %) Expression Expression Expression Expression C SiMn Ni Cr Cu N (1) (2) (3) (4) Comparative 0.06 0.4 1.1 8.1 18.2 0.1 0.049.22 9.27 5.07 18.00 Example 1 Comparative 0.05 2.0 9.5 0.1 16.0 2.00.13 6.52 12.03 92.39 0.02 Example 2 Comparative 0.08 1.0 7.0 0.5 15.01.0 0.16 6.19 11.48 125.22 10.64 Example 3 Comparative 0.08 0.5 6.0 0.515.0 1.0 0.16 5.72 10.73 137.92 13.05 Example 4 Comparative 0.06 1.0 7.00.2 15.0 2.0 0.17 6.04 11.42 109.54 9.29 Example 5 Comparative 0.06 2.08.0 0.2 15.0 2.0 0.19 6.81 13.28 83.00 6.15 Example 6 Comparative 0.061.0 8.0 0.2 15.0 1.5 0.20 6.96 12.09 102.08 12.38 Example 7 Comparative0.06 1.2 8.1 0.6 15.6 2.1 0.21 7.56 12.97 57.59 12.58 Example 8Comparative 0.06 1.7 8.9 0.2 16.0 2.0 0.21 7.53 13.68 55.53 9.23 Example9 Comparative 0.06 2.2 8.2 0.7 15.3 2.2 0.19 7.40 13.80 55.13 6.05Example 10 Comparative 0.30 1.0 6.0 0.2 16.0 0.0 0.18 5.72 16.83 46.4421.95 Example 11 Comparative 0.20 1.0 7.0 0.2 16.0 0.0 0.18 6.19 14.6384.54 17.54 Example 12 Inventive 0.08 1.0 8.9 1.0 16.0 1.0 0.20 8.1812.95 63.15 15.17 Example 1 Inventive 0.06 1.0 8.6 0.8 15.7 1.7 0.217.99 12.74 59.81 14.31 Example 2 Inventive 0.06 1.0 9.0 1.0 15.8 1.30.23 8.68 13.21 51.76 16.79 Example 3 Inventive 0.07 1.05 8.7 0.9 15.61.3 0.21 8.14 12.95 63.99 15.03 Example 4

Yield strength, tensile strength, and elongation of the each of thecold-rolled, annealed steel sheets of the inventive examples andcomparative examples were measured. Also, yield strength, tensilestrength, and elongation of skin pass-rolled steel sheets respectivelyprepared by skin pass rolling the cold-rolled, annealed steel sheetsaccording to the inventive examples and comparative examples by 20% weremeasured.

The measurement of the yield strength, tensile strength, and elongationwas carried out according to the ASTM standards, and the measured yieldstrength (YS, MPa), tensile strength (TS, MPa) and elongation (EL, %)are shown in Table 2 below. Also, occurrence of cracks in annealedmaterials was measured after a 180° adhesion bending test and resultsare shown in Table 2 below.

TABLE 2 Cold-rolled Skin pass-rolled Cracks steel sheet steel sheet byYS TS EL YS TS EL bending (MPa) (MPa) (%) (MPa) (MPa) (%) test (∘/x)Comparative 300.0 697.0 52.2 624.4 876.3 32.1 x Example 1 Comparative501.6 862.1 55 940.8 1205.3 20.2 ∘ Example 2 Comparative 379.7 1133.738.1 850.4 1517.5 14.9 ∘ Example 3 Comparative 311.4 1324.0 28.1 893.71652.2 11.9 ∘ Example 4 Comparative 362.9 958.3 39.8 920.3 1430.5 20.3 ∘Example 5 Comparative 435.9 1050.8 58.7 918.1 1397.8 26.1 ∘ Example 6Comparative 461.9 1028.2 50.3 953 1413.9 22.8 ∘ Example 7 Comparative427.7 878.8 59.7 802.8 1210.7 26.7 x Example 8 Comparative 457.6 88858.7 848 1220.2 27.7 x Example 9 Comparative 462.6 930.6 57.3 933.61332.6 23.7 ∘ Example 10 Comparative 508.7 948.2 32.2 881.1 1416.8 15.9∘ Example 11 Comparative 464.3 914.4 25.8 840.5 1313.5 12.3 ∘ Example 12Inventive 435.2 938.1 57.6 801.7 1288 25.5 x Example 1 Inventive 432.5878.7 59.5 841.8 1233.6 25.5 x Example 2 Inventive 453.1 876.6 60.1894.6 1261 25.3 x Example 3 Inventive 442.7 860.5 60.4 851.8 1233.1 26.3x Example 4

Referring to Table 2, in the case of Inventive Examples 1 to 4satisfying the composition of alloying elements suggested by the presentdisclosure and satisfying Expressions (1) to (4), it was confirmed thatthe cold-rolled, annealed steel sheets had yield strengths of 400 MPa ormore and elongations of 55% or more. In addition, referring to Table 2,the skin pass-rolled steel sheets of Inventive Examples 1 to 4 had yieldstrengths of 800 MPa or more and sufficient elongations of 25% or moreeven after skin pass rolling. In addition, it was confirmed that thesteel materials according to Inventive Examples 1 to 4 had pricecompetitiveness due to relatively low Ni contents of 1.0 wt % or less.

Referring to Tables 1 and 2, the steel materials according tocomparative examples will be evaluated.

The steel material according to Comparative Example 1, as a commerciallyavailable standard austenitic stainless steel, had a low yield strengthbecause the steel material did not satisfy the composition of alloyingelements of the present disclosure and Expressions (2), (3), and (4).Also, the commercial austenitic stainless steel of Comparative Example 1had inferior price competitiveness due to the high Ni content of 8.1 wt% which is far higher than that of the Ni content according to thepresent disclosure.

Because Comparative Example 2 does not satisfy Expression (1), aconsiderable amount of initial delta ferrite remains in the steelmaterial after cold rolling and annealing. Cracks easily occur at aninterface between delta ferrite phase and austenite phase during aforming process such as bending a steel material due to a phasedifference, and thus a low value of Expression (1) involves cracks whenbent. As a result, although Comparative Example 2 exhibited a high yieldstrength due to the high Si content and a high elongation, cracksoccurred by the bending test due to the remaining delta ferriteindicating inferior formability including bending characteristics.

All of the steel materials according to Comparative Examples 3 to 5 aresteel types not satisfying Expressions (1) to (4). Because Expression(1) was not satisfied, considerable amounts of initial delta ferriteremained in the steel materials after cold rolling and annealing, andthus formability including bending characteristics was inferior. Inaddition, because Expression (2) was not satisfied, low yield strengthswere obtained. In addition, because the value of Expression (3) exceeds100, plastic non-uniformity easily occurs due to phase transformationinto strain-induced martensite. In addition, due to the too low value ofExpression (4), serious dislocation pile-up occurred by planar slip. Asa result, elongation deteriorated. Particularly, elongations ofComparative Examples 3 to 5, which deteriorate because Expressions (3)and (4) were not satisfied, further deteriorated after skin passrolling, so that physical properties of the steel materials were notsuitable as skin pass-rolled steel sheets.

In Comparative Example 6, inferior formability including bendingcharacteristics was obtained because Expression (1) was not satisfiedand thus a considerable amount of initial delta ferrite remained in thesteel material after cold rolling and annealing. In addition, althoughthe steel material of Comparative Example 6 had the high yield strengthdue to the high Si content and Expression (2), the elongation was notsufficient due to effects of Expressions (3) and (4).

The steel material of Comparative Example 7 had inferior formabilityincluding bending characteristics because Expression (1) was notsatisfied and thus a considerable amount of initial delta ferriteremained in the steel material after cold rolling and annealing. Also,plastic non-uniformity easily occurs during deformation due to phasetransformation into strain-induced martensite because the value ofExpression (3) was over 100, which did not satisfy Expression (3).Therefore, the cold-rolled, annealed steel sheet and the skinpass-rolled steel sheet had inferior elongation.

The steel material of Comparative Example 8 satisfied the contents ofthe alloying elements except for Cu and satisfied Expressions (1) to(4). Thus, the cold-rolled, annealed steel sheet had excellent yieldstrength and elongation. However, Comparative Example 8 had inferior hotworkability due to an excessive Cu content. Evaluation thereof will bedescribed below in more detail with reference to Table 3.

The steel materials according to Comparative Examples 9 and 10 hadinferior hot workability due to excessive amounts of Si and Cu.Evaluation thereof will be described below in more detail with referenceto Table 3.

The steel materials according to Comparative Examples 11 and 12 hadinferior formability including bending characteristics due to aconsiderable amount of initial delta ferrite remaining in the steelmaterial after cold rolling and annealing because Expression (1) was notsatisfied. Also, plastic non-uniformity, in which stress is concentratedon weak parts of the steel materials, increased due to frequent crossslip in Comparative Examples 11 and 12 because the values of Expression(4) were too high. As a result, the cold-rolled, annealed steel sheetand the skin pass-rolled steel sheet had inferior elongation. Althougheffects of the stress concentrated by cross slip on elongation arenegligible in commercial steel materials, elongation significantlydeteriorate in high-strength steel materials having too high values ofExpression (2) as in Comparative Examples 11 and 12.

The austenitic stainless steel according to the present disclosure hasexcellent price competitiveness due to high productivity and high actualyield due to excellent hot workability. For comparative evaluation ofhot workability, reduction of area was measured in slabs of severalcomparative examples with high elongation and the inventive examples atdifferent temperatures. Measurement of the reduction of area wasperformed according to the ASTM standards by a high-temperature tensiletest, and results are shown in Table 3.

TABLE 3 Reduction of area at different temperatures (%) 800° C. 900° C.1000° C. 1100° C. 1200° C. Comparative 81.4 78.7 76.3 84.7 96.3 Example1 Comparative 40.3 43.6 53.4 66.6 88.2 Example 2 Comparative 42.5 50.569.5 88.1 95.2 Example 6 Comparative 52.8 57.2 69.3 82.4 95.2 Example 7Comparative 43.2 48.7 64.7 85.3 93.4 Example 8 Comparative 41.2 48.555.2 68.2 91.0 Example 9 Comparative 40.9 45.6 55.4 66.6 90.2 Example 10Inventive 53.3 64 75.1 88.7 96.6 Example 1 Inventive 50.1 54.7 71.1 87.197.0 Example 2 Inventive 60.0 56.9 66.2 84.5 95.8 Example 3 Inventive55.8 52.8 58.2 85.2 96.9 Example 4

Referring to Table 3, it was confirmed that reductions of area of 50% ormore were obtained at a high temperature of 800° C. or higher in thecase of Inventive Examples 1 to 4 satisfying the composition of alloyingelements suggested by the present disclosure and satisfying Expressions(1) to (4).

As a commercial standard austenitic stainless steel, the steel materialaccording to Comparative Example 1 had excellent hot workability due tolow amounts of Cu and N added to reduce the amounts of Si and Ni, whichare required to increase strength. However, a large amount of Ni, whichis an expensive element, is contained in the commercial 300 seriesaustenitic stainless steels, the 300 series austenitic stainless steelshave considerably low price competitiveness. In addition, as evaluatedin Table 2, the steel material had inferior yield strength because thecomposition of alloying elements and Expressions (2), (3), and (4) werenot satisfied.

In Comparative Examples 2, 6, 9, and 10, excessive amounts of Si wereadded to improve yield strength of the cold-rolled, annealed steelsheets and excessive amounts of Cu replacing Ni were added for pricecompetitiveness. The steel materials according to Comparative Examples2, 6, 9, and 10 had low hot workability due to excessive amounts of Siand Cu.

Because Si and Cu, which deteriorate hot workability, were added withinthe ranges suggested in the present disclosure, the steel materialaccording to Comparative Example 7 had excellent hot workability.However, as evaluated in Table 2, the steel material had inferiorformability because Expression (1) was not satisfied and had inferiorelongation of the cold-rolled, annealed steel sheet and the skinpass-rolled steel sheet because Expression (3) was not satisfied.

The Cu content of Comparative Example 8 exceeded the range suggested bythe present disclosure. Excessive Cu was segregated on edges or surfaceof slabs causing liquid metal embrittlement, thereby deteriorating hotworkability of Comparative Example 8. In Comparative Example 8, due toinferior hot workability, actual yield may decrease due to edge cracksoccurring after hot rolling, correcting costs therefor may increase, orinvestment for additional equipment to reduce edge cracks may berequired.

While the present disclosure has been particularly described withreference to exemplary embodiments, it should be understood by those ofskilled in the art that various changes in form and details may be madewithout departing from the spirit and scope of the present disclosure.

INDUSTRIAL APPLICABILITY

According to the present disclosure, a low-cost austenitic stainlesssteel having high strength and high formability applicable throughoutvarious industrial fields may be provided.

1. A low-cost austenitic stainless steel having high strength and highformability comprising, in percent by weight (wt %), greater than 0% andat most 0.08% of C, 0.2 to 0.25% of N, 0.8 to 1.5% of Si, 8.0 to 9.5% ofMn, 15.0 to 16.5% of Cr, greater than 0% and at most 1.0% of Ni, 0.8 to1.8% of Cu, and the remainder of Fe and other unavoidable impurities andsatisfying Expressions (1) to (4) below:Ni+0.47Mn+15N≥7.5  (1)23 (C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn≥12  (2)551−462(C+N)−9.2Si−8.1Mn−13.7Cr−29(Ni+Cu)≤70  (3)11≤1+45C−5Si+0.09Mn+2.2Ni−0.28Cr−0.67Cu+88.6N≤17  (4) (wherein C, N, Si,Mn, Cr, Ni, and Cu represent contents (wt %) of the elements,respectively).
 2. The low-cost austenitic stainless steel according toclaim 1, wherein a yield strength of a cold-rolled, annealed steel sheetis 400 MPa or more.
 3. The low-cost austenitic stainless steel accordingto claim 1, wherein an elongation of a cold-rolled, annealed steel sheetis 55% or more.
 4. The low-cost austenitic stainless steel according toclaim 1, wherein a yield strength of a skin pass-rolled steel sheet is800 MPa or more.
 5. The low-cost austenitic stainless steel according toclaim 4, wherein an elongation of the skin pass-rolled steel sheet is25% or more.
 6. A method for manufacturing a low-cost austeniticstainless steel having high strength and high formability, the methodcomprising: preparing a slab including, in percent by weight (wt %),greater than 0% and at most 0.08% of C, 0.2 to 0.25% of N, 0.8 to 1.5%of Si, 8.0 to 9.5% of Mn, 15.0 to 16.5% of Cr, greater than 0% and atmost 1.0% of Ni, 0.8 to 1.8% of Cu, and the remainder of Fe and otherunavoidable impurities and satisfying Expressions (1) to (4) below; hotrolling the slab to prepare a hot-rolled steel sheet and hot annealingthe hot-rolled steel sheet to prepare a hot-rolled, annealed steelsheet; cold rolling the hot-rolled, annealed steel sheet to prepare acold-rolled steel sheet and cold annealing the cold-rolled steel sheetat a temperature of 1050° C. or higher to prepare a cold-rolled,annealed steel sheet; and skin pass rolling the cold-rolled, annealedsteel sheet to prepare a skin pass-rolled steel sheet:Ni+0.47Mn+15N≥7.5  (1)23 (C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn≥12  (2)551−462(C+N)−9.2Si−8.1Mn−13.7Cr−29(Ni+Cu)≤70  (3)11≤1+45C−5Si+0.09Mn+2.2Ni−0.28Cr−0.67Cu+88.6N≤17  (4) (wherein C, N, Si,Mn, Cr, Ni, and Cu represent contents (wt %) of the elements,respectively).
 7. The method according to claim 6, wherein the skin passrolling is performed at a reduction ratio of 20% or more.
 8. The methodaccording to claim 6, wherein the slab has a reduction of area of 50% ormore at a high temperature of 800° C. or higher.